The invention relates to an optical communication system and optical calibration system using the optical source depolarization technology.
Optical transmission circuits using such optical amplifiers as optical fiber amplifiers and optical repeaters are extremely flexible to varying transmission rates, and are about to be put into practical application as long-distance trunk circuits. In general, however, optical amplifiers have a polarization dependency of optical gain. An example of this is that an optical fiber amplifier has a polarization dependency originating from the optical components composing the amplifier.
As the optical amplification multi-repeater system, whose development is now under way, for most part only amplifies the light at each repeater to compensate for the attenuation of signal light during transmission, the signal light distortion and optical noise that occur during transmission and at every repeater accumulate to the point of rendering a substantial effect on the transmission characteristics of the entire system.
Consequently, each optical amplifier used for the multi-repeater amplification is required to have an amplification characteristic measurement of 0.1 dB or less, or in other words extremely high precision and high resolution. However, because the insertion loss of various optical components used in the optical amplifier varies according to the state of the polarization of the signal light, and also because one uses such a calibration apparatus as an optical spectrum analyzer that has a polarization dependency of approximately 0.5 dB, it is extremely difficult to satisfy the ultra high precision and ultra high resolution requirements.
The technology of measuring at ultra high precision and ultra high resolution the amplification characteristics of each optical amplifier used in multi-repeater amplification is extremely important in constructing an optical communication system. Consequently, considerable amount of research is being carried out on this subject.
Being studied as a means of improving the calibration resolution and calibration precision of optical amplifier characteristics is the method of effectively eliminating the polarization dependency of optical amplifiers and calibration systems by depolarizing or reducing the degree of the polarization of a signal light whose degree of polarization is normally around 1 to reduce the optical power that is susceptible to the influence of polarization dependency of the calibration system and the calibration subject.
Degree of polarization (equals the level of polarization) is expressed by the ratio of the polarized component power to the total power of the light. In essence, degree of polarization of 1 is completely polarized light and degree of polarization of 0 is completely depolarized light.
FIG. 5 is a graph that shows the correlation between the inherent size of polarization dependency of a calibration system and calibration subject and the optical power displacement of polarization dependency that is observed when a light with a low degree of polarization is used. In the figure, the horizontal axis indicates the inherent size of polarization dependency of calibration system and calibration subject and the vertical axis shows the actually measured polarization dependency. The lines .alpha., .beta., and .gamma. in the graph have a degree of polarization of 0.1, 0.25 and 0.5, respectively. Each line represents the calibrations when signal light is used.
As FIG. 5 clearly shows, even when the inherent polarization dependency of the calibration system and calibration subject is 0.5 dB, if the degree of polarization is 0.1 (the polarized component is 10% of the entire signal light), that is the observed polarization dependency is approximately 0.05 dB. When the degree of polarization is 0.5 (the polarized component is 50% of the entire signal light), that is the observed polarization dependency is approximately 0.26 dB.
As can be seen here, the smaller the degree of polarization of the light used for the calibration, the smaller the observed polarization dependency. It is clear that calibration precision can be greatly improved by using a light whose degree of polarization is 0.5 or less.
Described below is the principle of depolarization that reduces the degree of polarization of a light with a large degree of polarization. The phase of an optical output from a laser is constant during a period that is commonly called "coherence time" (approximately the inverse of the full-width half-maximum linewidth of optical source), and beyond the coherence time a vibration whose phase is randomly different from the previous phase occurs. Because of this, as shown in FIGS. 6(a), (b), if a laser light is branched and a time difference far in excess of the coherence time is given to one light, a constant phase relationship will no longer exist between the divided lights and there will no longer be a correlation between the two.
As polarization is defined by the sum of orthogonal polarized components with a constant phase relationship, one cannot specify any single phase relationship. In essence, if orthogonally polarized lights with no correlation are mixed with equal power, the resulting light is in a polarization state that cannot be universally prescribed: in other words, the light is depolarized with a degree of polarization of 0. Here, if the orthogonally polarized components are not mixed by equal power, the difference will be polarized and the degree of polarization will be larger than 0 as the differential power.
Even when the orthogonally polarized components are mixed by equal power, if there remains even the slightest correlation between the orthogonally polarized components the polarized optical power will increase proportionately, and thus the degree of polarization will be larger than 0 as the ratio of correlation. Consequently, to generate a light with a small degree of polarization, it is necessary to minimize the correlation between orthogonally polarized lights, and mix them with the same power whenever possible.
One of the conventional optical depolarizing circuits using the above depolarization principle was the depolarizing optical element that realizes said depolarizing conditions by using the transmission rate differential (i.e., polarization dispersion) between orthogonally polarized lights in a highly birefringent optical fiber to generate between the orthogonally polarized lights a delay difference that exceeds the coherence time (e.g., Patent Journal 59-155806, IEEE Journal of Light Wave Technology Vol. LT-1, No. 3, pp 475-479).
To use such a depolarizer to depolarize the narrow laser light used for long-distance ultra high speed optical communication systems, with a linewidth of approximately 100 MHz or less and with a long coherence time, it is necessary to have an extremely long fiber since the polarization dispersion of the current polarization maintaining fiber is small, about 2 ps/m.
Attaching a circuit for gain stabilization to each optical amplifier has been used as a means of stabilizing signal light power and signal reception. This method is problematic, however, in that it requires complex optical repeaters and additional cost. One of the ways to stabilize signal light power without incorporating circuits in optical amplifiers is the technology of employing a polarization scrambler in the optical transmitter.
However, a polarization scrambler is ineffective unless the polarization state is randomized at a speed above the bit rate of the signal light. Therefore, it is necessary to employ such hardware as optical devices and electric circuits that respond at extremely high speeds. Further, it is extremely difficult for such hardware to accommodate high bit rates.
Moreover, in an optical amplification repeater system whose transmission circuits are made of optical fibers with small polarization dispersion, the optical fiber non-linearity creates a four wave mixing between the signal light and the optical noise generated by the optical amplifier. The four wave mixing then induces signal deterioration, which is a major constraint of optical amplifier repeater system performance.
Four wave mixing is a type of non-linearity optical phenomenon caused by the non-linearity of optical fiber. It refers to a phenomenon whereby the interaction between two light waves of different wavelengths generates a new signal light at a distance from the light waves equal to the difference in wavelengths between the two light waves. The generation efficiency of four wave mixing is proportional to the power of the two light waves with different wavelengths. The generation efficiency also increases as the optical fiber dispersion approaches 0.
The generation efficiency of four wave mixing is proportionate to the power of the relevant light waves. It is optimized when all the relevant light waves are in the same state of polarization, and it is nearly zero when they are in orthogonal state of polarization. When four wave mixing is generated between depolarized light, such as the optical noise generated from an optical amplifier, and signal light, its generation efficiency cannot be reduced even with the use of a polarization scrambler. Thus, this signal deterioration is unavoidable.
Assume we have a depolarizer that uses an optical path of two orthogonal polarization components formed with one conventional polarization maintaining fiber, corresponding to the principal axis of the fiber, to provide a time difference between the two orthogonal polarization components. Given the refractive index of the polarization maintaining fiber is approximately 1.5, an extremely long fiber length, approximately 4 km, is necessary to achieve the delay difference of approximately 7.3 ns between the orthogonal polarization components in the polarization maintaining fiber that is in turn necessary to achieve the 0.1 or less degree of polarization for a linewidth of 100 MHz. This creates the problems below.
1) As mode coupling occurs between the two orthogonal polarization components within the polarization maintaining fiber, the travelling time difference between the orthogonal polarization components inside the polarization maintaining fiber does not increase in proportion to the optical fiber length, and thus the degree of polarization obtained cannot be reduced. PA1 2) It is extremely difficult with the present fabrication technology to produce several kilometers of polarization maintaining fiber without a connection point. As mode coupling occurs between the orthogonal polarization components at connection points, the travelling time difference between the orthogonal polarization components inside the polarization maintaining fiber is reduced dramatically. PA1 3) As the polarization maintaining fiber is lengthened, the propagation loss difference between the orthogonal polarization components becomes considerable, and because the optical power between the cross polarized lights becomes different, it becomes even more difficult to reduce the degree of polarization obtained. PA1 4) Because the propagation loss of a polarization maintaining fiber of a few kilometers reaches a few dB, the signal light power of the polarization light obtained becomes small. PA1 5) A polarization maintaining fiber of a few kilometers is a considerable amount. It would require large hardware and would prove to be extremely expensive.
If the linewidth of the light is 1/10, the necessary fiber length would be ten times. Consequently, it has been practically impossible to realize a light wave depolarization circuit with conventional technology.